Capacitor for a semiconductor device and method for forming the same

ABSTRACT

A capacitor having high capacitance using a silicon-containing conductive layer as a storage node, and a method for forming the same, are provided. The capacitor includes a storage node, an amorphous Al 2 O 3  dielectric layer, and a plate node. The amorphous Al 2 O 3  layer is formed by a method in which reactive vapor phase materials are supplied on the storage node, for example, an atomic layered deposition method. Also, the storage node is processed by rapid thermal nitridation before forming the amorphous Al 2 O 3  layer. The amorphous Al 2 O 3  layer is densified by annealing at approximately 850° C. after forming a plate node, to thereby realize the equivalent thickness of an oxide layer which approximates a theoretical value of 30 Å.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a capacitor of a semiconductor device, and a method for forming the same.

2. Description of the Related Art

As semiconductor devices become more highly integrated, there is a necessity for reducing the area occupied by a capacitor, which in turn causes a reduction in the capacitance. To solve this problem, the structure of the capacitor is changed or a material having a high dielectric constant is employed. For instance, a method has been proposed for forming a capacitor using a Ta₂O₅ dielectric layer or a (Ba,Sr)TiO₃ (BST) dielectric layer having a dielectric constant higher than that of an oxide/nitride/oxide (ONO) layer structure typically used for a dynamic random access memory (DRAM).

However, the process is complicated. The structure of a typical capacitor is a silicon/insulator/silicon (SIS) layer structure in which polycrystalline silicon layers doped with an impurity are used for plate and storage nodes. However, a metal/insulator/silicon (MIS) layer or a metal/insulator/metal (MIM) layer structure is required for the case of using the Ta₂O₅ layer, and the MIM layer structure is required for the case of using the BST layer. That is, the structure of the capacitor must be changed.

If the Ta₂O₅ layer is used, then in order to overcome low step coverage, a layer must be formed using chemical vapor deposition (CVD) at a low temperature, which is a surface kinetic regime, and which may cause a deficiency in oxygen, leaving hydrocarbon residue in the layer or deterioration in crystallization. Thus, the dielectric constant is reduced, and insulating properties are poor. Accordingly, to overcome these problems, a dry O₂ annealing process at a high temperature is additionally required. Also, there has been disclosed a method for compensating for the insulating properties of the Ta₂O₅ layer by using an oxide layer under the Ta₂O₅ layer generated by a dry annealing process (Y. Ohyi, “Ta₂O₅ Capacitor Dielectric Material for Giga-bit DRAMs”, IEDM Tech. Dig., 1994. p831).

Meanwhile, diffusion is easily caused by discontinuities in the atomic arrangement at a grain boundary. Thus, when a thick oxide layer is formed to compensate for the leakage current properties of the Ta₂O₅ layer, diffusion of oxygen into the grain boundary is increased, to thereby oxidize a plate node. Accordingly, a reaction preventing layer is required between the Ta₂O₅ layer and the plate node of the capacitor to prevent reaction of the Ta₂O₅ layer with the plate node (U.S. Pat. No. 4,891,684).

In order to obtain excellent leakage current properties, a schottky barrier must be formed between a BST layer and an electrode. In order to form the Schottky barrier, an electrode should be formed of materials having a high work function, e.g., a metal (see Soon Oh Park, “Fabrication and Electrical Characterization of Pt/(Ba, Sr)TiO₃/Pt Capacitors for Ultralarge-scale Integrated Dynamic Random Access Memory Applications”, Jpn. J. Appl. Phys. Vol. 35,1996, pp. 1548-1552). In order to employ the metal electrode, an ohmic contact must be formed at an interface between the metal electrode and the polycrystalline silicon layer doped with an impurity. That is, an intermediate layer forming the ohmic contact must be formed and a barrier layer must be employed.

The material layer of a high dielectric constant, such as the Ta₂O₅ layer or the BST layer, requires a complicated process and structure, that is, a change of the structure of the capacitor to the MIM or MIS structure.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a capacitor of a semiconductor device, using a silicon-containing conductive layer as a storage node to increase capacitance.

It is another object to provide a method for forming a capacitor of a semiconductor device, using a silicon-containing conductive layer as a storage node to increase capacitance. Other and further objects will appear hereafter.

Accordingly, to achieve one objective, the capacitor of the present invention includes a storage node, a dielectric layer and a plate node. The storage node is a silicon-containing conductive layer such as a polycrystalline silicon layer doped with an impurity. Also, the storage node has a three dimensional structure selected from the group consisting of a stack type, a hemispherical grained silicon layer type and a cylinder type.

The dielectric layer is formed of amorphous Al₂O₃, on the storage node. Here, the amorphous Al₂O₃ layer is formed by transmitting vapor reactive materials supplied by each source to the storage node in which reactions are sequentially processed. The thickness of the dielectric layer is 10˜300 Å using an atomic layered deposition method, and the thickness of the amorphous Al₂O₃ dielectric layer is 40˜70 Å. Also, a reaction preventing layer is formed of one selected from the group consisting of silicon oxide, silicon nitride and silicon oxynitride layer.

The plate node is a conductive layer formed of polycrystalline silicon doped with impurities. Alternatively, the plate node is a conductive layer formed of a refractory metal, a refractory metal silicide material or a refractory metal nitride material. For example, the refractory metal may be W, Mo, Ta, Ti or Cr. Also, the refractory metal silicide material is formed by silicidation of a refractory metal silicide material, such as Wsi₂, MoSi₂, TaSi₂ or TiSi₂, among others. The refractory metal nitride material is formed by nitrification of a refractory metal, such as TiN.

To achieve another objective, a storage node is formed. The storage node is a polycrystalline silicon layer doped with an impurity, and the storage node has a three-dimensional structure selected from the group consisting of a stack type, a hemispherical grained silicon layer type and a cylinder type.

Then, a reaction preventing layer is formed on the storage node. The reaction preventing layer is formed by annealing the storage node at 300˜1200° C. In detail, a rapid thermal nitridation (RTN) process is performed using a N₂ source such as NH₃ gas as an ambient gas at approximately 900° C. Thus, the reaction preventing layer of the storage node may be formed of silicon oxide (SiO₂), silicon nitride (SiN) or silicon oxynitride (SiON).

Next, a dielectric layer is formed of an amorphous Al₂O₃ layer on the storage node. The amorphous Al₂O₃ layer is formed to a thickness of 10-300 Å by a method of supplying a reactive vapor phase material from each of several sources in sequence on a layer to be reacted with, i.e., the storage node, in which reaction, that is, deposition is performed in cycles, for instance, an atomic layer deposition (ALD) method. Preferably, the amorphous Al₂O₃ layer is formed to a thickness of 40-80 Å. The atomic layer deposition method is performed using one selected from the group consisting of Al(CH₃)₃ and AlCl₃ as an aluminum source, and the storage node is processed by hydrogen passivation treatment before performing the atomic layered deposition.

Then, a plate electrode is formed on the dielectric layer. The plate node is a conductive layer formed of polycrystalline silicon doped with impurities. Alternatively, the plate node is a conductive layer formed of a refractory metal, a refractory metal silicide material or a refractory metal nitride material. For example, the refractory metal may be W, Mo, Ta, Ti or Cr. Also, the refractory metal silicide material is formed by silicidation of a refractory metal silicide material, such as WSi₂, MoSi₂, TaSi₂ or TiSi₂, among others. The refractory metal nitride material is formed by nitrification of a refractory metal, such as TiN.

Also, a primary densification is performed on the amorphous Al₂O₃ dielectric layer, after the step of forming a plate node, by annealing the amorphous Al₂O₃ dielectric layer at a temperature below the temperature of crystallizing the amorphous Al₂O₃ layer, at 150-900° C. The annealing is performed using an ambient gas selected from the group consisting of O₂, NO and N₂ gas, or in a vacuum. Preferably, the primary densification is performed at 850° C.

Also, a secondary densification is additionally performed on the amorphous Al₂O₃ dielectric layer, before the step of forming a plate node, by annealing the amorphous Al₂O₃ dielectric layer at a temperature below the temperature of crystallizing the amorphous Al₂O₃ layer, at 150-900° C., using an ambient gas selected from the group consisting of O₂, NO and N₂ gas, or in a vacuum. Preferably, the annealing is performed using O₂ as an ambient gas at approximately 450° C.

Accordingly, the silicon-containing conductive layer is used as the storage node, and the capacitance can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objectives and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which:

FIG. 1 is a sectional view of a capacitor according to an embodiment of the present invention;

FIGS. 2 through 4 are sectional views showing structures of a storage node used for a capacitor according to embodiments of the present invention;

FIG. 5 is a sectional view for illustrating the step of forming the storage node on a semiconductor substrate;

FIG. 6 is a sectional view for illustrating the step of forming a dielectric layer on the storage node of FIG. 5;

FIG. 7 is a graph showing the relationship between the thickness of an equivalent oxide layer of an amorphous Al₂O₃ and applied driving voltage;

FIG. 8 is a graph showing leakage current density with respect to a drive voltage applied to a capacitor having an amorphous Al₂O₃ layer of 60 Å; and

FIG. 9 is a graph showing electrical characteristics of a capacitor according to various condition variables during annealing at approximately 450° C. after forming an amorphous Al₂O₃ layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present.

Referring to FIG. 1, a capacitor according to an embodiment of the present invention includes a storage node 200, a dielectric layer 400 and a plate node 500. The storage node 200 is electrically connected to an active region of a semiconductor substrate 100 through a contact hole formed in an interlevel insulating layer 150 covering the semiconductor substrate 100. A silicon-containing conductive layer such as a polycrystalline silicon layer doped with an impurity is used for the storage node 200.

An amorphous aluminum oxide Al₂O₃ layer is used for the dielectric layer 400 covering the storage node 200. The Al₂O₃ layer has little difference in dielectric constant between its crystalline phase, such as α-Al₂O₃ and γ-Al₂O₃, and its amorphous phase, and has a dielectric constant of about 10. However, the amorphous Al₂O₃ layer is more easily oxidized than silicon oxide, has a low permeability of alkali ions and excellent properties. Also, the amorphous Al₂O₃ layer has a smooth morphology and high resistance to diffusion through a grain boundary, and thus diffusion of oxygen through it can be suppressed.

The amorphous Al₂O₃ layer is formed by sequentially supplying a reactive vapor phase material from each of several sources on a layer, i.e., the storage node. Particularly, a whole amorphous Al₂O₃ layer is obtained by repeating the formation of thin films. For example, an atomic layer deposition (ALD) method may be used.

By forming the amorphous Al₂O₃ layer using the ALD method, supplying in sequence the reactive vapor phase materials on a layer where reaction for forming the Al₂O₃ layer is performed, high conformity may be obtained and a step coverage may reach about 100%. Few impurities remain in the amorphous Al₂O₃ layer due to process properties of the ALD method. It has been known that an Al₂O₃ layer formed by sputtering has poor step coverage and if the Al₂O₃ layer is formed by CVD, it is difficult to remove remaining impurities and form a thin layer. Accordingly, in a preferred embodiment, the amorphous Al₂O₃ layer is formed by the ALD method, and thus has high step coverage and a more amorphous state. The amorphous Al₂O₃ layer is formed to a thickness of 10-300 Å, preferably 40-80 Å.

Also, a reaction preventing layer 300 of silicon oxide (SiO₂), silicon nitride (SiN) or silicon oxynitride (SiON) may be further formed between the storage node 200 and the dielectric layer 400. However, since the amorphous Al₂O₃ layer has low diffusivity of oxygen, the reaction preventing layer 300 can be omitted. The plate node 500 formed on the dielectric layer 400 is formed of a conductive layer formed of a polycrystalline silicon doped with impurities. Alternatively, the plate 500 is formed of a conductive layer formed of a refractory metal, a refractory metal silicide material or a refractory metal nitride material. For example, the refractory metal may be W, Mo, Ta, Ti or Cr. Also, the refractory metal silicide material is formed by silicidation of a refractory metal silicide material, such as WSi₂, MoSi₂, TaSi₂ or TiSi₂, among others. The refractory metal nitride material is formed by nitrification of a refractory metal, such as TiN.

Meanwhile, the structures of a storage node of the capacitor may be formed in a variety of three-dimension structures. For instance, a stack type storage node 200 as shown in FIG. 1, can be adopted to the capacitor structure. Alternatively, a hemispherical grained silicon (HGS) layer may be formed on a surface of an electrode as shown in FIG. 2, so that the storage node 200 a has the HGS structure, which increases the surface area due to its irregularity. Other structures may be used, such as a cylinder type storage electrode 200 b as shown in FIG. 3, or a storage node 200 c as shown in FIG. 4 in which the hemispherical grained silicon layer is formed on the surface of the cylinder type electrode to increase the surface area. Even though three-dimensional storage nodes such as 200, 200 a, 200 b and 200 c are employed, a high conformity can be realized in the amorphous Al₂O₃ layer when the amorphous Al₂O₃ is formed by the ALD process. The high conformity avoids problems such as poor step coverage. Although preferred embodiment storage nodes 200, 200 a, 200 b and 200 c are illustrated, other structures are possible as would be well known to one skilled in the art.

Referring to FIG. 5, a storage node 200 is formed on a semiconductor substrate 100 where an interlevel insulating layer 150 is formed. The storage node 200 in the present embodiment may be shaped in a three-dimensional form such as a hemispherical grained silicon layer type or a cylinder type as described with respect to FIGS. 2 through 4, instead of the stack type. Also, the storage node 200 is formed of a conductive layer of a silicon group, such as a polycrystalline silicon layer doped with an impurity.

Then, the storage node 200 may be annealed by a rapid thermal process (RTP), to thereby additionally form a reaction preventing layer 300 covering the storage electrode 200. The annealing is performed at 300-1200° C., preferably 900° C., using an NH₃ gas as a nitrogen source for 60 sec, i.e., through rapid thermal nitridation (RTN). Through the annealing, the silicon of the storage node 200 reacts with nitrogen, to thereby form a SiN layer used as a reaction preventing layer 300. Also, a silicon oxide or silicon oxynitride layer may be used instead of the silicon nitride layer for the reaction preventing layer 300.

The reaction preventing layer 300 more completely prevents oxygen from being diffused into the storage 200 during the annealing process, which is performed later, using an ambient gas of oxygen. That is, the equivalent thickness (ET) of an oxide layer can be prevented from increasing due to diffusion of oxygen. However the silicon nitride layer need not be formed by the RTP. This is because low oxygen diffusivity is maintained by the Al₂O₃ layer.

Referring to FIG. 6, a native oxide layer remaining on the storage 200 is completely removed through hydrogen passivation treatment.

Then, the amorphous Al₂O₃ layer is formed on the storage 200 by a method in which each reactive vapor phase material from several sources is supplied step by step onto the storage node 200. This may be achieved in various ways, particularly by the ALD method. By the ALD method, an aluminum (Al) layer is formed on the storage 200 using an aluminum source, to a degree of atomic sized thickness. Then, the Al layer is oxidized with an oxidation agent to form an Al₂O₃ layer having an atomic-sized thickness, i.e., approximately 0.5-50 Å. Subsequently, the step of forming the Al₂O₃ layer having an atomic-sized thickness is performed in cycles, to thereby form an amorphous Al₂O₃ layer of 10-300 Å. Preferably, the amorphous Al₂O₃ layer is formed to a thickness of 40-80 Å.

Specifically, Al(CH₃)₃ or AlCl₃, preferably Al(CH₃)₃, is used as an aluminum source. Preferably, vaporized H₂O is used as the oxidation agent. When the step of oxidizing the Al layer using the oxidation agent is performed, the temperature of the semiconductor substrate 100 is 150-400° C., preferably approximately 350° C. Also, the amorphous Al₂O₃ layer is grown to a thickness of approximately 2 Å by each cycle. The amorphous phase of the amorphous Al₂O₃ layer is realized according to characteristics of the ALD process. Also, the Al₂O₃ layer has high conformity according to the characteristics of the ALD. Thus, a step coverage of approximately 100% can be realized.

Then, a plate electrode 500 as shown in FIG. 1 is formed on the dielectric layer 400. The plate electrode 500 is formed of a conductive layer formed of a polycrystalline silicon doped with impurities. Alternatively, the plate electrode 500 is formed of a conductive layer formed of a refractory metal, a refractory metal silicide material or a refractory metal nitride material. For example, the refractory metal may by W, Mo, Ta, Ti or Cr. Also, the refractory metal silicide material is formed by silicidation of a refractory metal silicide material, such as WSi₂, MoSi₂, TaSi₂ or TiSi₂, among others. The refractory metal nitride material is formed by nitrification of a refractory metal, such as TiN.

The grown amorphous Al₂O₃ layer has a refractivity of approximately 1.64λ, where A=633.0 nm. The amorphous Al₂O₃ layer can be densified through a subsequent annealing process. The degree of densification can be evaluated by measuring the refractivity and the thickness of the layer. That is, the amorphous Al₂O₃ layer is grown, and then annealed under an O₂ ambient gas, after that the refractivity of the annealed Al₂O₃ layer is measured to estimate the degree of the densification.

TABLE 1 Characteristics before and after growing Al₂O₃ layer Refractivity (λ = 633.0 nm) After growing 1.64  After annealing 1.692 (about 800° C., O₂, for 30 min)

As shown in Table 1, the refractivity of the amorphous Al₂O₃ layer is increased, through O2 annealing, which leads to densification of the amorphous Al₂O₃ layer. Thus, the dielectric constant of the amorphous Al₂O₃ layer is increased, so that the equivalent thickness (ET) of an oxide is minimized.

To show how the amorphous Al₂O₃ layer suppresses O2 diffusion, thickness of the SiO₂ layer formed on a bare wafer was measured as a function of the thickness of a layer of Al₂O₃ formed on the wafer. That is, an amorphous Al₂O₃ layer having various thicknesses was formed on a bare wafer. The wafer was treated with standard cleaners I and HF, and then the amorphous Al₂O₃ layer was annealed under an O2 ambient gas at approximately 800° C. Then, the thickness of the formed SiO₂ layer was measured using a spectroscopic elipsometer. The results are shown in Table 2.

As shown in Table 2, if the amorphous Al₂O₃ layer is not formed, the SiO₂ layer is grown to approximately 66.6 Å. If an amorphous Al₂O₃ layer is employed, the thickness of the SiO₂ layer is abruptly reduced. If the amorphous Al₂O₃ layer of approximately 100 Å is employed, the SiO₂ layer is reduced to a thickness of approximately 2 Å. As described above, the amorphous Al₂O₃ layer of the present embodiment suppresses O₂ diffusion, so that excellent capacitor properties can be realized even without the reaction preventing layer 300 being formed through the RTP. However, the reaction preventing layer 300 may be employed to completely prevent O₂ diffusion during a subsequent process of annealing.

TABLE 2 Thickness of SiO₂ layer -vs- thickness of Al₂O₃ layer Thickness of Al₂O₃ Thickness of SiO₂ after annealing (Å) (Å) 0   66.576 28.860 17.032 33.369 18.959 48.484 11.222 82.283  3.406 98.711  2.002 258.749   1.542

In order to increase the dielectric constant, a primary densification is performed on the amorphous Al₂O₃ layer, through annealing. This annealing for densification can be performed at any time after forming the amorphous Al₂O₃ layer, preferably after forming the plate node 500. The annealing is performed at approximately 150-900° C., preferably 850° C., which is lower than the crystallization temperature of the amorphous Al₂O₃ layer, using O₂ gas, NO gas or N₂ gas as an ambient gas, or in a vacuum. Preferably, the annealing is performed in the ambient gas of N₂ for approximately 30 min.

The amorphous Al₂O₃ layer gives an ET value which approximates, for instance, a theoretical ET value of about 26 Å, when the thickness of the amorphous Al₂O₃ layer is about 60 Å, and the dielectric constant of the Al₂O₃ is assumed to be 9. However, the annealing can be further performed to realize a value even more closely approximating the theoretical ET value. That is, a secondary densification is performed through annealing immediately after forming the amorphous Al₂O₃ layer, and this is a pre-treatment for the primary densification.

Here, the secondary densification is performed at 150-900° C., preferably 450° C., which is lower than the crystallization temperature of the amorphous Al₂O₃ layer, using O₂ gas, NO gas or N₂ gas as an ambient gas, or in a vacuum. Preferably, the secondary densification is performed by annealing in the ambient gas of O₂ for about 30 min.

Table 3 shows various electric characteristics measured under various conditions in order to illustrate the effect of the secondary densification. That is, the storage 200 is formed from a polycrystalline silicon layer on a semiconductor substrate 100, the amorphous Al₂O₃ layer is formed, and electrical properties of the capacitor are measured. Table 3 contains the results of ten measurements, showing the effects when the RTN process of NH₃ is performed at 900° C. for 60 sec or not performed, the amorphous Al₂O₃ layer is formed to 60 Å or 300 Å, and the secondary densification is performed using O₂ at approximately 450° C. for 30 min, at approximately 800° C. for 30 min, or not performed.

As shown in Table 3, the Al₂O₃ layer of about 300 Å has a leakage current density of 20 nA/cm² or less, regardless of other conditions.

However, the leakage current of the Al₂O₃ layer of about 60 Å is changed by other conditions. For example, in the case No. 7 of the Al₂O₃ layer of 60 Å without RTN, and with the secondary densification at 800° C., the ET is approximately 57 Å, which is the greatest for all cases where the thickness of the Al₂O₃ layer is 60 Å. Also, for example, in case No. 1 with the RTN process, the ET is approximately 47 Å, which is more than the theoretical ET value 30 Å (i.e., an RTN layer of 4 Å+an Al₂O₃ layer of 26 Å). This means that the equivalent oxide layer has grown.

Moreover, in case No. 3 when the Al₂O₃ layer of 60 Å thick, the temperature of the secondary densification is 450° C., and the RTN is performed, the ET is 40 Å, and in the case No. 6 when the Al₂O₃ layer is 60 Å thick, the secondary densification temperature is 450° C. and the RTN process is not performed, the ET is 37 Å. However, in the case No. 6, the leakage current density of approximately 700 nA/cm² is higher in comparison to approximately 45 nA/cm² in the case No. 3.

From these results, we note that the electric characteristics of a capacitor are excellent in the case of performing secondary densification at 450° C. as, for example, in case No. 3. Also, if the reaction preventing layer 300 such as a SiN₂ layer, a SiO₂ layer or a SiON layer is formed through annealing such as RTN, between the storage node 200 and the dielectric layer 400, the electrical characteristics of the capacitor are enhanced.

TABLE 3 Electric characteristics of capacitor as a function of secondary densification leakage Thickness of Thickness of Temperature of Capacitance current equivalent Al₂O₃ layer O₂ secondary (pF) density oxide layer C_(min)/C_(max) No. RTN (Å) densification C_(min) C_(max) tan δ (nA/cm²) (ET) (Å) (%) 1 yes  60 800 615 661 0.012 25.8  47 93 2 300 800 235 247 0.031 ≦20 125 95 3  60 450 718 772 0.009 45.77  40 93 4 300 450 259 263 0.042 ≦20 117 98 5 300 — 218 229 0.045 ≦20 135 95 6 no  60 450 766 832 0.06  704.38  37 92 7  60 800 535 546 0.014 37.32  57 98 8 300 450 248 252 0.019 ≦20 117 98 9 300 — 210 219 0.052 ≦20 141 96 10  300 800 222 223 0.046 ≦20 138 99

As described above, when the amorphous Al₂O₃ layer is employed as the dielectric layer 400 and the secondary densification is performed, the electrical characteristics of the capacitor are enhanced. However, the ET values realized under the above conditions do not reach the theoretical value of 30 Å. Thus, the effect of performing the primary densification after forming the plate node 500 will be described. That is, the plate node 500 is formed under conditions as shown in Table 3 and then annealed at approximately 850° C. for 30 min using an ambient gas of N₂, for a primary densification, and then the electrical characteristics of the capacitor are measured for the ten different process conditions of Table 3.

Table 4 shows the results of these measurements. In Table 4, ‘C’, ‘B’, ‘T’, ‘L’ and ‘R’ indicate center, bottom, top, left and right portions of a wafer, respectively. The primary densification can reduce the ET values for all cases as shown in Table 4. In the case that the Al₂O₃ layer is annealed at approximately 450° C. during the secondary densification process and RTN is performed (No. 3-C), the ET value is 35 Å. In the case where the other conditions are the same as that of No. 3-C, but the RTN process is not performed (No. 6-C), the ET value is 31 Å. Both results approximate to the theoretical value of 30 Å. Thus, these results show that the secondary densification of the amorphous Al₂O₃ layer further increases capacitance.

TABLE 4 Electric characteristics of capacitor depending on primary densification Leakage Thickness of Position of Thickness of Temperature of Capacitance current Equivalent C_(min)/C_(max) semiconductor Al₂O₃ layer O₂ secondary (pF) density oxide layer (%; No. RTN substrate (Å) densification C_(min) C_(max) tan δ (nA/cm²) (ET; Å) 2 V) 1 yes C  60 800 680 690 0.012  7.96 44.8  98.6 2 C 300 800 250 270 0.019 15.2 118.3  92.6 3 T  60 450 830 874 0.014 68.4 35.07 95   C 840 890 0.028 28.6 34.89 94.4 B 830 876 0.032 25.6 35.31 94.7 L 825 859 0.035 53.6 36.02 96   R 840 882 0.014 65.4 35.07 95.2 4 C 300 450 270 290 0.017 16.5 110    93.1 5 C 300 — 260 270 0.017 13.1 116.1  96.3 6 no T  60 450 932 987 0.019 — 31.34 94.4 C 930 990 0.017 677   31.30 94.4 B 905 946 0.011 — 32.71 95.7 L 925 971 0.014 — 31.86 95.3 R 910 964 0.016 — 32.09 94.4 7 C  60 800 540 580 0.018  2.29 53.4  93.1 8 C 300 450 310 313 0.018 13.3 98.8  99   9 C 300 — 250 260 0.02   4.89 119.8  96.2 10  C  60 800 230 240 0.02  11.2 133.5  95.8

FIG. 7 shows the ET at the amorphous Al₂O₃ layer of 60 Å in Table 4, as a function of an applied voltage. That is, reference numerals 710, 715, 730 and 735 indicate ET values for the cases No. 1, No. 7, No. 3 and No. 6 respectively of Table 4, as a function of applied voltage. As shown in FIG. 7, if the secondary densification using O₂ is performed at 450° C. for 30 min and the primary densification of N₂ is performed at 850° C. for 30 min (730 and 735), i.e., in the cases No. 3 and No. 6 of Table 4, the measured ET values approximate the theoretical ET value of 30 Å.

Referring to FIG. 3, reference numerals 810, 815, 830 and 835 indicate leakage current densities for the cases No. 7, No. 1, No. 6 and No. 3 respectively of Table 4, as a function of applied voltage. As shown in FIG. 8, among the cases 730 and 735 approximating the theoretical ET value as illustrated in FIG. 7, i.e., in the cases No. 3 and No. 4 of Table 4, case No. 3 exhibits a lower leakage current density at driving voltages of 2V or less. Thus, if the secondary densification with O₂ is performed at approximately 450° C. for 30 min according to the conditions of case No. 3 of Table 4, i.e., after the RTN process, and the primary densification with N₂ is performed at 850° C. for 30 min, then the electrical characteristics of the capacitor are excellent.

The effect of the secondary densification will be described as follows. The temperature for the secondary densification is set to 450° C., which can produce excellent electrical characteristics of a capacitor as described above. Also, whether the ambient gas for the secondary densification is a gas other than O_(2,) (i.e., N₂), or whether the secondary densification is performed, are set as variables. The time for the secondary densification and the thickness of the amorphous Al₂O₃ layer are also set as variables. The ET value and the leakage current density are measured as these variables are changed. The conditions are shown in Table 5.

TABLE 5 Time for Thickness of secondary Reference Al₂O_(3 layer) Ambient densification numeral (Å) gas (min) 910 40 N₂ 10 920 50 O₂ 10 930 50 O₂ 30 940 50 N₂ 10 950 60 O₂ 10 960 60 O₂ 30 970 60 N₂ 10 980 60 none  0

FIG. 9 shows the ET and leakage currents for the various conditions shown in Table 5. Reference numeral 940 represents the lowest ET and the highest leakage current density. However, reference numerals 950, 960, 970 and 980, for an amorphous Al₂O₃ layer of approximately 60 Å, represent ETs and leakage current densities which are similar to each other. Also, as the amorphous Al₂O₃ layer is closer to 40 Å (910), the ET values are similar and the leakage current density is increased. Thus, in the amorphous Al₂O₃ layer of approximately 60 Å, the properties of capacitor change little without regard to whether the secondary densification is performed or not, to what ambient gas is used, or to changes in secondary densification time. This means that the secondary densification is not always a required step. In other words, the secondary densification is for compensating for the primary densification. Even though the plate node 500 is formed on the dielectric layer 400 without the secondary densification, a capacitor having excellent characteristics can be realized. That is, when the amorphous Al₂O₃ layer and the plate node 500 are formed after an RTN process of the storage node 200, and then the primary densification using N₂ at approximately 850° C. for 30 min is performed, the electrical characteristics of the capacitor are excellent.

Accordingly, the amorphous Al₂O₃ dielectric layer is formed on the storage node using a method for supplying each reactive vapor phase material from several sources on a layer i.e., the storage node, where a reaction, e.g., deposition, is performed, in cycles, e.g., ALD method. During forming the amorphous Al₂O₃ layer, the amorphous phase can be realized using the method for supplying reactive vapor phase materials on a layer where a reaction is performed in cycles. Also, by such a method, no impurity remains in the amorphous Al₂O₃ layer. The amorphous Al₂O₃ layer has a low O₂ diffusivity. Thus, when a conductive layer of the silicon group is employed as the storage node, it is possible to prevent the equivalent oxide layer from growing excessively. That is, a capacitor of the SIS structure can be realized, like in the capacitor of the ONO structure, to thereby overcome difficulties due to a change of the structure of the capacitor to the MIS structure or MIM structure.

Also, an amorphous Al₂O₃ layer having a smooth morphology and high conformity can realize a step coverage of approximately 100%. Thus, the storage node can be formed as a cylinder type, an HSG type or a stack type, to thereby increase capacitance. The amorphous Al₂O₃ layer has a dielectric constant equivalent to that of an aluminum layer of a crystalline phase, to thereby realize high capacitance.

After forming the plate node on the amorphous Al₂O₃ layer, the amorphous Al₂O₃ layer can be densified by annealing at a temperature below the temperature of crystallizing the Al₂O₃, e.g., at approximately 850° C. The densification reduces the thickness of the amorphous Al₂O₃ layer and increases the refractivity. That is, the dielectric constant of the amorphous Al₂O₃ layer is increased. Also, the thickness of the equivalent oxide layer is reduced, so that a thickness approximating the theoretical ET value, e.g., approximately 30 Å, can be realized, to thereby increase capacitance.

A reaction preventing layer can be formed between the amorphous Al₂O₃ layer and the storage node, in order to reduce the thickness of the equivalent oxide layer and help the capacitor operate stably. A secondary densification may be additionally performed to compensate for the step of densifying the amorphous Al₂O₃ layer, so that the thickness of the equivalent oxide layer approximates the theoretical ET value, to thereby increase capacitance.

Preferred embodiments of the invention have been disclosed in the drawings and specification, and although specific terms are employed, they are used in a descriptive sense only and not for purposes of limitation. The scope of the invention is set forth in the following claims. 

What is claimed is:
 1. A method for forming a capacitor of a semiconductor device, comprising: (a) forming a storage node; (b) forming a dielectric layer of amorphous Al₂O₃, on the storage node; and (c) forming a plate node on the dielectric layer, wherein a primary densification is performed on the amorphous Al₂O₃ dielectric layer, after forming the plate node, wherein a secondary densification is additionally performed on the amorphous Al₂O₃ dielectric layer, before forming a plate node, and wherein the secondary densification is performed by annealing the amorphous Al₂O₃ dielectric layer at a temperature below the temperature of crystallizing the amorphous Al₂O₃ layer.
 2. The method of claim 1, wherein the secondary densification is performed at 150-900° C.
 3. The method of claim 1, wherein the secondary densification is performed using an ambient gas selected from the group consisting of O₂, NO and N₂ gas, or in a vacuum. 